Labeling of biomolecules is frequently performed in biological assays. Such labeling studies have been widely used to elucidate structural and/or functional properties of various biomolecules, including carbohydrates, lipids, nucleic acids, nucleotides and proteins. Enzymes are of particular interest because they catalyze fundamental biochemical reactions within living organisms. For example, DNA and RNA polymerases assist in genomic replication and transcription by catalyzing the polymerization of nucleotides into nucleic acids.
Conventional labeling techniques generally involve the attachment of one or few organic labels comprising fluorescent small molecules, e.g., dyes, to the biomolecule of interest. However, such labeled conjugates are generally not suitable for use in single molecule assays due the toxicity effect of the label on the biomolecule, and/or the poor detectability (as characterized, for example, by low signal/noise ratio, brightness, e.g., quantum yield, signal lifetime, etc) and photostability of such conjugates. There is therefore a need in the art for labeled biomolecule conjugates that emit stronger and more stable signals than is feasible with conjugates produced by conventional labeling methods, and that retain sufficient biological activity for use in single molecule assays.
Disclosed herein are improved labeled biomolecule conjugates, as well as novel methods of making and using such conjugates. Such conjugates comprise labeled biomolecules exhibiting improved biological activity, detectability and/or photostability and that are suitable for use in single molecule assays. In some embodiments, the conjugates comprise a biomolecule linked to multiple dye labels that retain sufficient biological activity for use in single molecule assays.
The superior detectability of the conjugates of the present disclosure permits a wide range of powerful new approaches not hitherto feasible using conventional labeling methods, including, for example, extended imaging of biological samples over an extended period of time, real time in situ visualization of biomolecules or biomolecular activity in vivo or in vitro, optical coding of biomolecules, physical manipulation of biomolecules and/or biomolecular sorting, all of which can optionally be performed in high-throughput format.
For example, disclosed herein are labeled polymerase conjugates comprising a polymerase linked to a label that emit signals of superior intensities and durations, thus improving their performance in single molecule sequencing applications. In some embodiments, the labeled polymerase conjugates include multiple dyes (typically three or more) linked in tandem to a single polymerase without significant loss of polymerase activity. In other embodiments, the labeled polymerase conjugates comprise a nanoparticle label that typically emits stronger and more stable signals relative to conventional organic dyes.
The labeled polymerase conjugates provided herein can undergo FRET with an acceptor-labeled nucleotide bound to the active site in such a manner that the resulting FRET-based signal is readily detectable in a single molecule system, and also emit signals of sufficient duration to permit longer “reads” from a single nucleic acid molecule, thus permitting single molecule reads of increased length and accuracy. Such conjugates also retain high levels of polymerase activity, thus increasing the efficiency of single molecule sequencing systems using such conjugates.
The production of such improved conjugates is associated with several technical challenges. For example, biomolecules labeled with nanoparticles frequently exhibit a high degree of aggregation; it can also be difficult to precisely control the ratios at which the biomolecule will attach to the nanoparticle, a problem compounded by the difficulty of determining the stochiometric composition (i.e., ratio of biomolecule to nanoparticle) of the resulting conjugates. Similarly, while the detectability of conjugates comprising organic dye labels can be improved by increasing the number of dye labels linked to the biomolecule, such increased dye loading is typically accompanied by a reduction or loss in activity of the biomolecule. There remains a need in the art for labeled biomolecule conjugates exhibiting reduced aggregation and increased biomolecular activity along with superior detectability. There is also a need for improved methods for conjugating biomolecules, e.g., proteins, to labels wherein the stochiometry of the conjugated components can be reliably controlled and the activity of the biomolecule preserved.